Because of the daily limits placed on earning Mark of the World Tree, this will take a character a few months to earn.

The item itself that teaches the mount is attached to a letter sent to a character's mailbox immediately upon earning the previously mentioned achievement.

From: Malfurion StormrageSubject: A Gift from the Arch Druid

Never again will Hyjal shudder beneath the endless forces of flame. Never again shall the mortal races of the world tremble before the minions of Ragnaros.

Now, thanks to your efforts, they will tremble before US.

Your heroism on the Molten Front has been instrumental in the fight against the Firelord. You have gathered allies from all over Kalimdor and turned them into an army. You have fought the minions of Ragnaros... and prevailed. And for that, you have our deepest thanks.